Advances in polymer chemistry and technology over the last few decades have enabled the development of high-performance polymeric fibers. For example, liquid-crystalline polymer solutions of rigid-rod polymers can be formed into high strength fibers by spinning liquid-crystalline polymer solutions into dope filaments, removing solvent from the dope filaments, washing and drying the fibers; and if desired, further heat treating the dried fibers to increase tensile properties. One example of high-performance polymeric fibers is para-aramid fiber such as poly(paraphenylene terephthalamide) (“PPD-T” or “PPTA”).
Many other examples of polymers capable of producing high-performance fibers are known and many such polymers and copolymers are soluble in concentrated sulfuric acids. A common solvent like sulfuric acid is a preferred solvent for achieving liquid crystalline concentrations of such polymers and provides fiber manufacturing processes with attractive economics. The processing of polymers from sulfuric acid solutions, however, can result in residual amounts of sulfur being left in the processed fibers in the form of undesirable impurities or incorporation into the polymer itself.
It has been determined that removal of sulfur in as spun or intermediate fibers processed from sulfuric acid can improve certain yarn physical properties, and thus accurate methods for determining sulfur content are desirable. An accurate determination of sulfur content in fibers, however, is difficult. In some cases, especially at low sulfur concentrations, traditional combustion/gas chromatography may lack precision and or reproducibility and require more sample measurements for improved confidence. As such, an improved method is needed in the art.